When the Baggot Inn first re-opened a few months ago with this service you had to 'prepay' your pump i.e. register your credit card at the bar and authorise say €20 worth of booze, after which the pumps cut off.
I'm not sure if this is still the case though.
It was possible to evade this. About 8 months ago I was there on a work night out that I had organised. They tried to insist on my card being used but I refused. Too easy for people to abuse and not easy to find out who had what later. Somebody else put up a card but it was that or just use the bar like normal.